Chain microstructure of two in-reactor alloy impact polypropylene resins with same stiffness and different toughness

It is crucial to establish a relationship between chain microstructure and mechanical performance for the ultimate applications of impact polypropylene. Here, two in-reactor alloy impact polypropylene resins (samples A and B) with same stiffness and different toughness are fractionated into nine fractions (at 35, 80, 100, 105, 110, 115, 120, 125, 140 °C, respectively) using preparative temperature rising elution fractionation (P-TREF). These fractions are characterized with high-temperature gel permeation chromatography (HT-GPC), Fourier transform infrared (FTIR) spectrometer, 13 C nuclear magnetic resonance ( 13 C NMR) spectrometer, differential scanning calorimetry (DSC), and thermal fractionation. It is found that both samples are primarily made up of isotactic polypropylene homopolymer, ethylene-propylene rubber (EPR), ethylene-propylene segmented and blocky copolymers. The content and M w of ethylene-propylene rubber in sample A (16.90 wt%, 500.0k) are higher than those (15.78 wt%, 385.4k) of sample B. Thus, sample A shows better toughness. The crystallinities of the fractions 4–9 in sample A (50.5, 51.0, 60.8, 52.7, 47.2, 48.5%) are higher than the corresponding fractions of sample B (47.8, 49.0, 51.0, 49.1, 46.6, 41.6%). Particularly, the content of high-temperature fractions (115–140 °C, i.e. fractions 6–9) is slightly higher than those of the fractions in sample B, but their molecular weights of the corresponding fractions are lower. Finally, the comprehensive effect leads to the same stiffness of the two samples. Definitely, the above results provide guidance on chain structure for improving mechanical properties of polypropylene with excellent stiff-tough balance.